1. Field of the Invention
This invention relates to an inductive heating apparatus, and more particularly to an inductive heating apparatus wherein energy is stored in a resonant circuit.
2. Description of the Prior Art
Various types of inductive heating devices have been proposed. For example, in U.S. Pat. No. 4,074,101, a pair of inversely polarized, parallel-connected silicon controlled rectifiers (SCR) are connected in parallel with a resonant circuit including an induction heating coil and a capacitor. This circuit generates an electromagnetic field about the heating coil when the SCR's are alternately caused to conduct so that a metal pan which is electromagnetically coupled with the heating coil is heated by eddy currents induced therein. However, the SCR's are often damaged by surge voltages caused by an abrupt change of load during heating.
To eliminate such defects, Japanese published application No. 51-128742 to Tokyo Shibaura Denki K.K. teaches an inductive heating device using first and second serially connected switching transistors, with a serial heating coil and capacitor connected in parallel with the second transistor. A power supply is connected across the pair of transistors. In this apparatus, currents alternately flow in a first loop consisting of the power supply, the first transistor, the heating coil and the capacitor, and in a second, resonant, loop consisting of the second transistor, heating coil and capacitor.
In this arrangement, the transistors must never conduct simultaneously, since the resulting short circuit will damage the transistors. To obviate this problem, this Japanese application teaches that after each conduction period, neither transistor should conduct for a period. Thus, after the current flowing through the heating coil ceases, a specific time period lapses before the other transistor begins to conduct. Although the transistors will not conduct simultaneously, there is still a problem, specifically, that heating efficiency is lowered when neither transistor is conducting.